1. Field of the Present Invention
The present invention relates to a shift control apparatus for an automatic transmission, which performs a shift operation of the transmission in accordance with the driving state of an automotive vehicle.
2. Description of the Related Art
An automatic transmission selects a speed range by selectively engaging various friction devices (e.g., clutches and brakes, etc.) and performs the speed change (the up-shift speed change and the down-shift speed change) to obtain any other speed (the position of the gear) by engaging or disengaging the friction devices. It is a conventional practice to predetermine a plurality of shift patterns corresponding to various parameters related to the driving state of the vehicle, such as vehicle speed, throttle opening, temperature of transmission oil and the like, to select a particular shift pattern which is appropriate for a current driving state, and to perform the shift control on the basis of the selected pattern.
There is known an automatic transmission provided with a mode change-over switch adapted to be selected and operated by a driver, which includes an "AUTO" position, a "POWER" position and a "SNOW" position. When the "POWER" position is selected, the shift pattern is fixed to an acceleration-oriented "power pattern". When the "SNOW" position is selected, the shift pattern is fixed to a "snow pattern" which is suitable for a snowy road. When the "AUTO" position is selected, i.e., when neither "POWER" position nor "SNOW" position is selected, the shift pattern is automatically switched to either one of the above-mentioned "power pattern" and an economy-oriented "economy pattern" in accordance with the driving state. Such an automatic transmission is disclosed, for example, in "Service Manual for NISSAN RE5R01A-Type Full-Range Electronically-Controlled Automatic Transmission" page A-7, issued in March 1989 by Nissan Motor Co., Ltd, the assignee of this application.
There is also known an automatic transmission having an ordinary-temperature shift pattern and a high-temperature shift pattern, which can be selected depending upon the temperature related to the transmission or the engine, typically the transmission fluid temperature. In this instance, an automatic transmission controller constantly monitors the transmission fluid temperature and prevents it from being extremely elevated by performing a switching from the ordinary-temperature shift pattern to the high-temperature shift pattern when the transmission fluid temperature becomes higher than a predetermined temperature. The relationship between the ordinary-temperature shift pattern and the high-temperature shift pattern substantially corresponds to the relationship between the above-described "economy pattern" and "power pattern", in that the shift point of the high-temperature shift pattern as compared to that of the normal-temperature shift pattern is set to a higher speed side to perform a shifting operation at a higher speed under the same conditions of the vehicle speed and the transmission fluid temperature. Such an automatic transmission is disclosed, for example, in Japanese Patent Laid-open Publication No. 62-74,726.
In the prior art automatic transmission that performs switching between a plurality of shift patterns in accordance with the transmission fluid temperature, for suppressing heat generation within the torque converter in a high temperature condition of the transmission fluid, the current shift pattern for normal temperature condition shown in FIG. 4A is switched into another shift pattern for high temperature condition shown in FIG. 4B. By this, the region for driving at a lower speed range (e.g., the second or third speed range in a four-speed automatic transmission) is extended as compared to the shift pattern for the normal temperature condition. Therefore, an increase in the transmission fluid temperature may involve an unintended shifting operation based on newly selected shift pattern, irrespectively of increase in the vehicle speed when depressing an accelerator pedal during a high-load driving, such as a up-hill driving, thereby giving unpleasant feel to the driver and/or passengers. This becomes marked upon occurrence of unintended shift operation skipping an intermediate speed range, e.g., from the top or fourth speed range to the second speed range in the case of a four-speed automatic transmission.